Oxidization of hydrocarbon compounds through molecular oxygen, and particularly oxidation through air have been examined for many years, and numerous methods have been disclosed. In the autoxidation of hydrocarbon compounds, the oxidation of cyclohexane is particularly important from an industrial perspective. The obtained cyclohexanone and cyclohexanol are very important compounds as raw materials for nylon 6 and nylon 6,6.
The oxidation of hydrocarbon compounds through molecular oxygen progresses via the corresponding hydroperoxide. The selectivity of an oxidation reaction from a hydrocarbon compound to a hydroperoxide is high, but under oxidation reaction conditions, hydroperoxides decompose into various byproducts other than the targeted alcohol and ketone, and therefore the selectivity decreases. In order to prevent this decrease in selectivity, a method is generally adopted in which the oxidation reaction is ended at a stage in which the conversion is low, and the hydroperoxides in the reaction solution that remain without decomposing are decomposed at the next step to produce the target ketone and/or alcohol.
Methods for decomposing hydroperoxides can be broadly categorized into two types of methods. The first is a method which adds a small amount of a transition metal compound or the like to decompose the hydroperoxide, and the second is a method which causes contact with an alkaline aqueous solution to decompose the hydroperoxide.
Of these, with the first type of method, the rate of decomposition of the hydroperoxide is slow, and the decomposition selectivity is also not very high, and therefore ordinarily the second type of method is adopted.
Note that with the process of the second type of method, the neutralization of carboxylic acid produced as a byproduct by the oxidation reaction, and the hydrolysis of esters produced as byproducts by the same oxidation reaction are simultaneously performed, and therefore this is known as a saponification step.
In the saponification step, hydroperoxide is decomposed with high selectivity, and therefore it is crucial that hydroperoxide be moved rapidly to the alkaline water phase. This is because radical decomposition of hydroperoxides having low selectivity progresses in the oil phase, and the decomposition selectivity decreases. With the saponification step, hydroperoxides are rapidly moved to the water phase, and detached, and therefore a strong alkaline aqueous solution such as alkali metal hydroxides or the like is used. However, the carboxylic acids produced in the oxidizing step reduce the pH of the saponification step. In order to prevent this, a method is widely adopted in which the saponification step is divided into two steps: a carboxylic acid neutralization step and a hydroperoxide decomposition step, and the oxidation reaction solution and the alkaline aqueous solution are brought into contact. This is called a two-step saponification process. However, the alkaline water phase that is discharged from the carboxylic acid neutralization step has a low pH value, and therefore this cannot be recirculated and used.
It is also difficult to recycle and reuse the strong alkali. Namely, with the two-step saponification process, the consumption of alkali necessary for at least the neutralization of carboxylic acid and the hydrolysis of ester in the process cannot be avoided.
On the other hand, if an alkali metal carbonate such as sodium carbonate is used as the alkali, the alkali metal carbonate can be regenerated and recycled by combusting the alkaline water phase that is discharged. However, the pH of the alkali metal carbonate aqueous solution is significantly lower than the pKa (inverse logarithm of the acid dissociation constant of 12˜13) of the hydroperoxide, and there is almost no effect of causing the hydroperoxide to dissociate and move to the alkaline water phase. Accordingly, with saponification through an alkali metal carbonate aqueous solution, the decomposition rate of the hydroperoxide is slower and the decomposition selectivity is lower compared to two-step saponification using a strong alkali. Moreover, the ketone/alcohol ratio of the ketone and alcohol that are produced is also lower than that of the two-step saponification using a strong alkali, and when the purpose is to obtain a ketone, the production of a ketone through the dehydrogenation of an alcohol becomes necessary, which is also disadvantageous.